Design and installation of a novel wetland Carp harvesting set up at Lake Bonney, South Australia

During 2009–2010, Lake Bonney (near the township of Barmera in SA) received 26 gigalitres of environmental water from the Murray River. It was anticipated that Carp (Cyprinus carpio) would accumulate in large numbers at the lake inlet as water was delivered (Fig1), providing a unique opportunity to trial a wetland Carp separation cage (WCSC) for controlling the estimated 50–100 tonnes of this species in the lake, as well as a number of designs for screening fish.Although numerous types of screens have been used to restrict the movement of fish either into or out of wetlands, most do not achieve the best environmental outcome in terms of allowing the free passage of native fish and other fauna while restricting the movement of Carp and other unwanted species.

Project aim and methods: Fishing/tagging activities and monitoring in the lake proper were undertaken in association with delivery of an environmental watering to Lake Bonney, and installation of a prototype wetland carp separation cage, to evaluate:

The response of Carp and native fish during the provision of environmental water, and therefore the need to accommodate the passage of large-bodied native fishes during future water allocations; and

The species diversity, abundance and size structures of captured fish (Carp and large-bodied fishes)

Two new carp exclusion screens (jail bars with 31mm apertures between the bars and square grid-mesh with 44 x 44 mm internal dimensions) (Fig 2) were trialled in the culverts to evaluate:

their effect on flow velocity; and,

whether an angle-mount and the high flow-velocities in the culvert would combine to clear the screens by pushing debris towards the water’s surface (and potentially over the top of the screen).

Findings: Scientific sampling and commercial fishing activities within the lake and inflow point, combined with fish tagging, allowed estimation of the resident population of several large-bodied fish species (native and alien), and their response to inflow. The size of the resident adult Carp population was estimated via a Peterson mark-recapture tagging experiment at 44,606 individuals. A similarly large but unquantified biomass of Bony Herring was also detected. Otherwise, only three large Freshwater Catfish and two Golden Perch were recorded, suggesting the lake’s large-bodied native fish population is very low (with the exception of Bony Herring).

Carp were observed to aggregate in large numbers around the inflow point, and spawning activity was observed within 24 hrs. Their efforts to exit the lake via the culverts was blocked by the carp screen. In contrast, relatively few large Bony Herring and no other large-bodied native fish were captured near the inflow point, however thousands of juvenile Bony Herring were observed in January 2010 when Carp were absent.

Significant refinements to strengthen Carp screens; enable them to pivot; and, prevent public access were required to enable carp screens to operate without fouling with debris, and to prevent vandalism. When set to an angle of ~33° fouling and flow constriction was significantly reduced. Most entrained fish and turtles were also able to pass over the top of this design.

Lessons learned and future directions: Although the cage operated according to its intended design and function during the 2010 trial, some operational issues were observed, necessitating refinements that have resulted in a pragmatic, adaptable and safe device.

Fixed screens such as grid mesh and the ‘jail bar’ design should not be used at wetlands like Lake Bonney that have high flows and easy public access, because:

impeding Carp movement is inefficient and often obstructs native species

regular maintenance is required

they tend to deteriorate over time, and can be easily vandalised

they can compress Carp into wetlands (ie juvenile Carp pass through a screen and grow to a point where they cannot move out though the screen).

While commercial fishing can be a valuable tool for controlling Carp, it is of limited use as a ‘stand alone’ technique as netting a proportion of adult fish does not stop Carp from spawning.

The level of by-catch (356 Bony Herring, as well as a few Golden Perch, Goldfish (Carrassius auratus) and Birds) signals the need to survey the resident native fauna on a site-by-site basis prior to installing any Carp management infrastructure. Also, the motivation of Carp to migrate out of the lake decreased over time, suggesting that harvesting should occur in the early stages of the lake being filled.

Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy.

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